The linear solenoid described in, for example, JP2015-84395A is known for a linear solenoid that generates driving force in the axial direction. The linear solenoid in JP2015-84395A includes a cylindrical plunger that is supported movably in the axial direction. This plunger is disposed inward of a cylindrical magnetic delivery core, and is disposed outward of a columnar center core.
The space between the other axial end of the magnetic delivery core and the other axial end of the center core is closed by a flange part that is provided at the center core. Thus, the volume of space surrounded with the magnetic delivery core, the center core, the plunger, and the flange part needs to be change to displace the plunger in the axial direction. Explanation will be given below with this space referred to as a first space. In JP2015-84395A, a breathing passage that communicates in the axial direction is provided for the plunger to enable the volume change of the first space.
Specifically, the plunger in JP2015-84395A includes a generally cylindrical bushing inward thereof. This bushing is supported slidably by the outer peripheral surface of the plunger. A flange-shaped increased diameter part that spreads toward the outer diameter side is provided integrally with the bushing disclosed in JP2015-84395A at the other axial end of the bushing. When the plunger is displaced toward the other axial end, this increased diameter part is brought into contact with the flange part to function as a stopper for restricting the displacement of the plunger toward the other axial end.
Despite the above, as a first issue, the increased diameter part blocks the inside of the breathing passage. The responsiveness of the plunger accordingly deteriorates.
As a second issue, the contact area between the increased diameter part and the flange part is large when the increased diameter part is in contact with the flange part. The large contact area between the increased diameter part and the flange part causes a sticking phenomenon at the contact part between the increased diameter part and the flange part at the time of displacement of the plunger toward one axial end. The responsiveness of the plunger accordingly degrades. The sticking phenomenon is a phenomenon in which some time is taken for the fluid to flow into the contact part even though the contact part is quickly separated, thus making difficult the quick separation of the contact part.
In JP2015-84395A, there is proposed an art for limiting the sticking phenomenon by radially forming many grooves at the flange part to make small the contact area between the increased diameter part and the flange part. However, forming many grooves at the flange part causes cost increases. In other words, there is an issue of responsiveness deterioration if many grooves are not formed at the flange part.